In one conventional mode of "inching" of a vehicle the transmission is kept in gear and the brakes are manually modulated by use of a foot pedal. This is undesirable because it requires considerable operator effort and because the service brakes experience a relatively rapid rate of wear. In another well known mode, when the vehicle's service brakes are applied by the depression of a foot pedal an associated disc and plate type transmission clutch is caused to slip so that the transmission is substantially neutralized. This is most often accomplished by the brake system fluid circuit acting upon an "inching" valve disposed between a pressure source and the clutch, as has been widely used in shuttle-type lift trucks. This is desirable since the vehicle's ground speed is reduced for more precise control, while the speed of the engine is maintained at a relatively high rate to enable a rapid response of the auxiliary equipment operated by the engine. However, controlled slip at a fixed setting of the manually operated control member in both of these modes of operation has heretofore been impractical.
The interconnection between the service brake pedal and a clutch pedal can be done mechanically, but this requires rather critical adjustments to function properly U.S. Pat. No. 2,972,906 issued to C. S. Schroeder on Feb. 28, 1961, for example, discloses a left foot pedal that mechanically operates a spool of a valve for decreasing the pressure delivered to a clutch in order to controllably slip it. A right foot pedal is independently depressible to solely engage the service brakes, and the left foot pedal is mechanically connected to the right foot pedal after some free travel of the left foot pedal to depress it and to cause the application of the service brakes after some clutch slippage.
U.S. Pat. No. 3,181,667 issued to K. R. Lohbauer et al. on May 4, 1965 is illustrative of another double pedal system for automatically neutralizing a vehicle's transmission with the application of the service brakes. Depression of the right brake pedal causes application of the service brakes, whereas depression of the left brake pedal causes actuation of a transmission neutralizing valve associated with the transmission to disengage it while simultaneously cooperating with a cross shaft arrangement to physically move the right brake pedal and cause the brakes to be applied after a certain amount of free travel.
At the same time, hydrodynamic torque converters having bladed impeller, reactor and turbine elements have been widely incorporated in vehicles between the engine and a multispeed transmission. U.S. Pat. No. 3,820,417 issued to T. E. Allen et al. on Jun. 28, 1974 illustrates a more sophisticated variation thereof wherein a disc type input clutch is disposed within the rotating housing of the torque converter to controllably deliver power from the engine to the impeller element. That patent further discloses a disc type lockup clutch engageable at a relatively high torque converter output speed for directly mechanically connecting the input rotating housing and the turbine element and obtaining an improved efficiency of the drive train. To effectively absorb the energy peaks resulting from the release and engagement of the downstream clutches of the transmission, the converter input clutch in U.S. Pat. No. 3,820,417 is disengaged during each gear shift and controllably reengaged after certain selected ones of the transmission clutches were engaged. The design of the transmission clutches could thus be simplified because they would not be required to absorb the full energy levels of the Shift. On the other hand, the control system associated with the actuating pistons of the input clutch and the lockup clutch was solely of the hydraulic valving type and not fully responsive to the full range of operating conditions of a vehicle.
U.S. Pat. No. 3,680,398 issued to R. C. Schneider et al. on Aug. 1, 1972 illustrates another torque converter with a hydraulically modulated or slippable input clutch which has a hydraulic control valve mechanism for preventing the impeller element from being driven in a reverse direction during shifts of the transmission under certain conditions of movement of the vehicle. Specifically, a preselected minimum pressure level is directed to the actuating piston of that input clutch to prevent the impeller from reversing direction and to thus reduce the power level transmitted through it. A radially oriented valve spool within the impeller element is sensitive to the speed of rotation thereof, and substantial efforts are made to control the dumping and filling of the input clutch with respect to the transmission clutches during different shifting conditions.
U.S. Pat. No. 3,822,771 issued to S. A. Audiffred et al. on Jul. 9, 1974 discloses another hydraulic control system for a torque converter of the type described that has a dual purpose option. For example, the operator of a wheel loader can adjust the controls to apportion the power delivery to the wheels and to auxiliary equipment to better match the work tasks being undertaken. More particularly, a right foot pedal could be manually depressed to controllably increase the pressure level to the actuating piston of the converter input clutch and to deliver more torque to the wheels. A knob on the dash could be adjusted by the operator to set a limit to the amount of torque being delivered to the wheels and to minimize the amount of tire slippage. Another knob could be adjusted to change over the control system so that the depression of the same right foot pedal would cause solely the acceleration of the engine with a fully engaged converter input clutch. In that control system a left foot pedal solely engaged the service brakes, and a central foot pedal sequentially engaged the service brakes and neutralized the transmission.
U.S. Pat. No. 3,621,955 issued to J. B. Black on Nov. 23, 1971 is illustrative of torque delivered through the converter input clutch to the wheels. For example, when a wheel loader is forcing a bucket forwardly into a pile of earth the input clutch could be controllably slipped below a preselected output speed of the torque converter in order to minimize tire slippage and to thus reduce the amount of tire wear.
The aforementioned torque converter equipped drive trains have not been fully commercially exploited because the control systems used therewith have become too complex in an attempt to accomplish all of the tasks required to controllably shift the converter input clutch, the lockup clutch, and the speed and directional clutches of the transmission in the desired manner. Moreover, many of these systems have not been sufficiently adjustable to make the systems practical to matching a wide range of vehicular operating conditions.
The broad adaptation of automated electronic-over-hydraulic transmission controls of the type shown by U.S. Pat. No. 4,208,925 issued Jun. 24, 1980 to R. G. Miller et al.; U.S. Pat. No. 4,414,863 issued Nov. 15, 1983 to D. L. Heino; U.S. Pat. No. 4,699,239 issued Oct. 13, 1987 to T. Ishino et al.; and U.S. Pat. No. 4,734,861 issued Mar. 29, 1988 to R. B. Berolasi et al., indicates that they have become fully accepted and even demanded by progressive vehicular operators. The electronic portion of these controls is capable of being programmed to accomplish a wide variety of logic steps relatively instantaneously after receiving the input signals from a plurality of signal generating devices. Thereafter, the electronic portion directs control signals to a plurality of solenoid-operated valves for directing fluid to the various clutches controlling the transmission gear ratios in a fully automated manner, in a fully manually selected manner, or in a combination of the two forms of control.
What is needed is an electrohydraulic control device for controllably operating an input clutch of a vehicular drive train that would incorporate a microprocessor-based electronic control module responsive to the manual movement of a control member by the vehicle operator. The subject electrohydraulic control device should electrically operate a valve and thereby precisely control the degree of engagement of the input clutch by supplying pressurized fluid thereto, and should contain logic routines for automatically overriding the manually called for degree of engagement under certain conditions of operation of the vehicle. Preferably, the drive train should include a torque converter with an impeller element driven by the input clutch, a turbine element, and a lockup clutch for mechanically bypassing the torque converter, and be especially adaptable for use in an earthmoving vehicle. In such instance the subject device should be responsive to the Speed of the engine, the output speed of the torque converter, and changes in the transmission gear selector to controllably manage the degree of engagement of the input clutch and the lockup clutch in accordance with preprogrammed logic and sequence steps. Specifically, it would be desirable to avoid excessive engine lug on the one hand to maintain the responsiveness of auxiliary equipment operated directly by the engine, and to avoid engine overspeeding on the other hand. Furthermore, the device should be integrated into a fully practical braking system for the vehicle, and be of a construction sufficient to fully overcome one or more of the problems associated with the prior art as set forth above. Moreover, the device should improve the overall productivity of the vehicle on which it is mounted, and preferably should reduce fuel consumption.
The present invention is directed at solving one or more of the problems as set forth above.